This invention relates to a method for driving a print head in an ink jet printer for ejecting ink drops to record characters and pictures, and an ink jet printer.
An ink jet printer is a printer of such a type that ink drops are ejected from narrow nozzles arrayed on a print head and are caused to reach a recording medium such as paper, thus recording characters or pictures in the form of dots. This ink jet printer is characterized by a high recording speed, a low recording cost and easy realization of color print.
The print head in the ink jet printer is a so-called serial head which is shorter than the page width of paper, or a so-called line head with a large length which is substantially the same dimension as the page width of paper. The system for ejecting ink drops is a piezo system using a piezoelectric element, or a thermal system using a heating element.
The above-described line head need not be moved in the direction of the page width of paper by driving means such as a motor as in the serial head at the time of recording. Therefore, the line head needs no driving means and it is easy to realize miniaturization of the printer body and reduction in cost. In the thermal system, it is possible to increase the number of driving elements for ejecting ink drops and the array density of the driving elements, more easily than in the piezo system. Therefore, the thermal system is more advantageous to realization of a line head. For these reasons, an ink jet printer having a line head of a thermal system is proposed.
However, the thermal system has drawbacks such as a lower energy efficiency and a larger dissipation power in recording than the piezo system. To overcome these drawbacks, in the ink jet printer, a time-division driving system in which a plurality of heating elements employed in a serial head of the thermal system are divided into several blocks, each of which is driven in a time-divisional manner, must also be applied to a line head of the thermal system.
The ink jet printer generally uses digital image processing by a so-called dither method or an error diffusion method in expressing the gradation. In these methods, however, since the gradation is expressed theoretically using a plurality of dots, the actual resolution is lowered. Moreover, since the rough or granular appearance remains due to the visibility of dots, the picture quality is lowered. Therefore, in the ink jet printer, it is necessary to reduce the rough or granular appearance and to improve the picture quality by reducing the diameter of dots and increasing the array density of dots.
The reduction in diameter of dots can be realized by reducing the size of the heating elements, the diameter of the nozzles and the volume of the chamber, and thus reducing the volume of ink particles to be ejected, with respect to both the line head and the serial head of the thermal system. However, the increase in array density of dots is more difficult with the line head of the thermal system than with the serial head.
Specifically, with the serial head of the thermal system, the array density of dots can be increased relatively easily by raising the ejection frequency of ink drops or reducing the head scanning rate in the head scanning direction, and by reducing the paper feed pitch in the direction of the paper feed direction. On the other hand, with the line head of the thermal system, the array density of the heating elements must be increased in the page width direction, while the array density of dots can be increased in the paper feed direction by a method similar to that of the serial head. This causes increased difficulty of working and assembly of the line head, lower yield, increased scale of the head drive circuit, and hence larger cost and lower reliability.
Moreover, with the serial head of the thermal system, the unevenness in the quantity of ejection of ink drops (dot size and print density) and in the ink drop position is made less visible by a so-called multipath system for recording one line with a plurality of nozzles, often in the high definition mode. On the other hand, with the line head of the thermal system, such a technique cannot be adopted since recording is completed by one scanning operation. Therefore, with the line head of the thermal system, it is important how the unevenness in the quantity of ejection (dot size and print density) for each nozzle and in the ink drop position can be restrained for high definition.
In view of the foregoing status of the art, it is an object of the present invention to provide a method for driving a print head in an ink jet printer which enables realization of an ink jet printer capable of providing a recorded image of a high picture quality with less rough or granular appearance at a high speed, and such an ink jet printer.
According to the present invention, there is provided a method for driving a print head in an ink jet printer in which ink drops are ejected from a plurality of nozzles and dropped on a recording medium so that information including a character and/or an image is recorded in the form of dots based on the ink drops. The method comprises the steps of causing the print head having a driving element for ejecting ink drops from the nozzles to scan the same portion on the recording medium only once in one print, and driving the print head to modulate the diameter of a dot by the number of ink drops, using one or a plurality of ink drops for forming one dot.
In such a method for driving a print head in an ink jet printer according to the present invention, the print head is driven to modulate the diameter of a dot by the number of ink drops.
According to the present invention, there is also provided an ink jet printer for ejecting and dropping ink drops from a plurality of nozzles onto a recording medium and recording information including a character and/or an image in the form of dots based on the ink drops. The ink jet printer has a print head having a driving element for ejecting ink drops from the nozzles. In the ink jet printer, the print head is caused to scan the same portion on the recording medium only once in one print and is driven to modulate the diameter of a dot by the number of ink drops, using one or a plurality of ink drops for forming one dot.
In such an ink jet printer according to the present invention, the print head is driven to modulate the diameter of a dot by the number of ink drops.